Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Harihar Rajaram

Second Advisor

Donald O. Whittemore

Third Advisor

James J. Butler, Jr.

Fourth Advisor

Roseanna Marie Neupauer

Fifth Advisor

John Scott McCartney

Abstract

Riparian evapotranspiration (RET) is an important component of basin-wide evapotranspiration (ET), especially in subhumid to semi-arid regions, with significant impacts on water management and conservation. A common method of measuring ET is using the eddy correlation technique. However, since most riparian zones are narrow, eddy correlation techniques are not applicable because of limited fetch distance. Techniques based on surface-subsurface water balance are applicable in these situations, but their accuracy is not well constrained. In this study, we estimated RET within a 100 meter long by 45 meter wide riparian zone along Rock Creek in the Whitewater Basin (1,100km) in central Kansas using a subsurface water balance (SSWB) approach. The soils were comprised largely of silty loam and clay, with gravel fragments at higher depths. Deeper alluvial sediment in the riparian zone is primarily clay that acts as an aquitard overlying the bedrock and overlying this layer is more permeable alluvial sediment and soil. The soil moisture contribution to RET is expected to be large in this setting. The SSWB approach was based on a monitoring network that included six soil moisture profilers with capacitance sensors at 4-6 depths (during different periods of the study), 4 water table wells, a deep bedrock well, and a weather station.

Continuous data collection and analysis extended from July 2010 to December 2013. The RET estimates obtained from the different profile locations vary significantly, even though they typically exhibit coherent trends. Results are presented on an annual and inter-annual (three periods within the year) basis. This variability results from the highly heterogeneous soils in the vadose zone (2-3 m thick), where soil moisture (rather than groundwater) is the major source of water for riparian vegetation. Variable vegetation density and species also likely contribute to the variability.

The water-balance based approach is very cost-effective, even with the large number of soil moisture sensors deployed. However, the high degree of variability in RET estimates from individual profiles even within a relatively short reach, suggests caution in the application and interpretation of water balance based techniques in vadose zones with natural vegetation, unless large sensor networks are used.

Comments

Sixth advisor: Holly R. Barnard.

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